select createtime, sum(new_users) over (order by createtime) as new_users
from (
select createtime::date as createtime, count(user_id) as new_users
from "user"
where createtime between '2015-12-03 00:00:00' and CURRENT_DATE
group by createtime
) t
order by createtime asc;

user is a reserved keyword and needs to be quoted, otherwise you can't use it as an identifier.